I've converted plastic and metal figures in the past, but how practical is it to try to convert resin figures? The resin cast models and parts that I've dealt with in the past were pretty brittle. So what's the best practice with resin figures and modifying them?

It really depends on the resin used. Careful cutting with a razor saw, or using a dremel with a rotary burr of a bit to grind out what you don't need. (be sure to wear a dust mask)